In recent years, there has been a rapid proliferation of radio frequency (RF) devices. In diverse arenas such as communications, security, audio/video, home and vehicular use, and beyond, the desire for wireless technologies has spurred the development of many products that utilize radio frequency transmitters.
Examples of the growth in technology in this field include U.S. patent application Ser. No. 11/531,018 for a “Method and Apparatus for Identifying Unused RF Channels,” and U.S. patent application Ser. No. 11/690,747 for an “FM Transmitter for an MP3 Player.” The disclosures of these patent applications are herein incorporated by reference.
The radiation output of radio frequency devices is limited by local regulatory law. In the United States, for example, RF devices are regulated by the Federal Communications Commission, whose rules are codified in Sections 2, 4, 5, 15, and 18 of Title 47 of the Code of Federal Regulations. For example, RF devices operating in the FM radio spectrum are regulated by Section 15.239, in which “intentional radiators” operating in the 88-108 Mhz frequency band are limited to a field strength of 250 microvolts/meter at 3 meters within a 200 kHz band centered on the operating frequency.
RF devices typically include both an RF transmitter and an attached antenna. In such devices, RF transmitter power must be adjusted so that when transmission occurs in combination with the antenna, the overall radiation output of the RF device does not exceed the legal limits. If the RE device is capable of operation at multiple frequencies, then RF transmitter power must be set so that radiation output is compliant for all operating frequencies. Because antenna gain varies with frequency, prior art devices have complied with radiation output limits by setting RF transmitter power to the maximum allowable at the operational frequency at which antenna gain is the highest (i.e. the frequency at which antenna efficiency is greatest). However, this means that for all other operational frequencies, radiation output is compromised, as radiation output necessarily falls below the legal limit due to the antenna's lower efficiency at these other frequencies. Thus, prior art devices fail to maximize radiation output throughout their range of operating frequencies.
The present invention solves the aforementioned problems in the prior art by providing a system and method whereby radiation output is maximized within allowable limits for all operational frequencies regardless of variation in antenna gain across the operational frequency range.